1. Field of the Invention
This invention relates to an improvement of an electromagnetically operated shutter of the type having a coil arrangement in a plane in parallel with a sector ring. The ring is rotated with a Lorentz force generated by applying to the coil arrangement a magnetic field in the direction of the photographing optical axis, the rotation of the sector ring being arranged to open and close a set of shutter blades.
2. Description of the Prior Art
Camera shutters, that have shutter blades opened and closed by the rotation of a sector ring, include a type in which the sector ring is rotated by a group of permanent magnets arranged to form a magnetic field in the direction of an optical axis. The rotation force (dyne) of the sector ring of this electromagnetically operated shutter is determined by the magnetic flux density Bg (gauss) of the magnetic field produced by the magnet, a driving current I (ampere) which is arranged to flow to the coil, and the length of an effective portion of the magnetic field of the coil, i.e., in the coil winding, the length Lu (meter) of the winding component in the radial directions centering on the rotation axis of the sector ring. The rotating force F can be expressed by: ##EQU1## The rotating force F, therefore, can be made greater by increasing either the magnetic flux density Bg, the current I or the effective length Lu of the coil. However, the limited space available within the camera, and the limited capacity of the power source in the camera impose a restriction on any attempt to increase the magnetic flux density Bg and the current I. Particularly in the case of an electromagnetically operated shutter to be incorporated in a compact camera, it is therefore necessary to have the length of the effective portion of the coil increased as much as possible while the magnets to be used must have an efficient shape and must be efficiently arranged.
Further, in an electromagnetically operated shutter of the type arranged to be urged by a spring in the direction of closing, and then to be closed by the force of the spring after it is opened, the shutter is charged by a return spring to a maximum degree when the shutter is fully opened. Therefore, assuming that the electromagnetic driving force remains constant throughout the process of opening the shutter, the difference between this electromagnetic driving force and the urging force of the return spring becomes minimal when the shutter is fully opened.
Such being the arrangement of the shutter, when a power source for the electromagnetic operation deteriorates, an adverse effect of this deterioration; becomes noticeable in the vicinity of full opening of the shutter. In other words, either the shutter blades do not open to a prescribed maximum degree, or the shutter blades behave unstably in their full open state. Conversely, in the initial stage of opening the shutter, the urging force of the return spring is weak. Accordingly, the shutter blades are then greatly accelerated by the electromagnetic driving force. Therefore, even when the application of electromagnetic force is cut off by a control circuit, the shutter is not promptly closed and thus the responsiveness of the shutter becomes unsatisfactory.